It is known that there are three types or modes of heat transfer, namely conduction, convection, and radiation. All electric resistance heaters utilize one of those forms of heat transfer in order to supply heat to the surrounding environment. In general, electric resistance heaters have a heat generating element (e.g. a resistance wire) that is coupled to a source of electrical energy. When the electrical energy is supplied to the resistance wire, the wire will heat up due to its resistance. The amount of heat produced by the resistance wire is a factor of the wire material and shape, and the voltage, current and/or frequency of the electrical energy supplied thereto.
Generally, in electric resistance heaters, the resistance wire is surrounded by and/or minimally in contact with a sheath material. The sheath material also contributes to the operating characteristics of the heater.
It is also known to have electric heaters that utilize quartz for the outer sheath material even though quartz is considerably more expensive to use as compared to more common heater sheath materials such as metals or ceramics. There are many reasons for utilizing quartz, including:
1. Quartz can endure high temperature use. PA1 2. Quartz is relatively transparent to infrared energy which allows the heat generating element inside the quartz to radiate heat directly from the element to the process or load with little elevation in temperature of the quartz. PA1 3. Quartz is considered to be one of the few acceptable materials for use in specialty environments or processes such as ultra pure semiconductor processing, e.g. heating deionized water. PA1 4. Quartz has a low thermal coefficient of expansion which inherently gives it the ability to withstand significant thermal shock and temperature excursions without fracturing. PA1 5. Quartz has reasonably good resistance to corrosion when exposed to many chemicals and deionized water. PA1 6. Quartz is typically a fused glass material with a very small molecular spacing. It is thus possible to fabricate sealed heaters that do not "breath" or allow contaminants around them to penetrate therethrough and attack the heating element, nor allow materials liberated by the heating element from contaminating the process or surrounding environment.
However, while there are known electric resistance heaters that utilize quartz as the outer sheath material, the configuration of such prior art heaters generally dictate that they function as radiant heaters (in the radiant mode of heat transfer) and not as convective or conductive heaters (respectively the convective mode of heat transfer and the conductive mode of heat transfer). This situation exists because the prior art quartz heaters do not substantially heat the quartz itself as is needed for convection and conduction type heating to occur. As such, the prior art electric resistance quartz heaters do not take advantage of the many characteristics of quartz as a sheath material and thus do not operate as convection or conduction mode heaters. This limits the scope of applications in which the heater may be used.
In U.S. Pat. No. 3,047,702 entitled Plate Heater, issued to F. L. Lefebvre on Jul. 31, 1962, there is disclosed a plate heater that utilizes quartz. A resistance element formed as a coil is retained against a surface of a quartz plate such that portions of the coil are in contact therewith. However, because most of the heating surface of the helixes of the resistive coil is not in contact with the quartz, there is little heating of the quartz. Rather than transferring heat to the quartz plate, the heating coil heats up the surrounding medium. Thus the '702 plate heater generally only operates in a radiant heat transfer mode making the heater rather inefficient and/or limiting its use to lower temperature heating applications.
In U.S. Pat. No. 4,531,047 entitled Clip-Mounted Quartz Tube Electric Heater, issued to Canfield et al. on Jul. 23, 1985, there is disclosed an electric heater which includes a quartz tube having a heater coil therein. The heater coil is supported by a ceramic support that extends the length of coil and is formed with a heat reflecting groove. Small arcuate portions of each helix of the heater coil are in contact with the inner surface of the quartz tube. The '047 patent recognized that prior art quartz heaters such as the Lefebvre '702 patent were deficient as indicated above and thus tries to alleviate the deficiencies by adding a supporting heat reflecting member to concentrate the heat developed within the tube by the heating coil.
In view of the above, it is an object of the present invention to provide a more efficient quartz heater.
It is another object of the present invention to provide a quartz heater that can operate in any one or all three of the three heat transfer modes.
It is yet another object of the present invention to provide an electric, resistance element type heater having a quartz sheath in which the quartz sheath supplies heat in the convection or conduction heat transfer mode.